Large area, light responsive, avalanche photodiodes are described in the IEEE Transactions on Nuclear Science, VOL NS-30, 1 February 1983 on page 431 et seq. in an article entitled "Scintallation detectors using large area Silicon Avalanche Photodiodes" by G. Huth et al and in the IEEE Transactions on Nuclear Science, VOL NS-32, NO. 1, February 1985 on page 563 et seg. in an article entitled "Recent Advances In Large Area Avalanche Photodiodes" by M. R. Squillante et al.
Those articles describe a large area (one inch diameter) avalanche photodiode for use with scintallation crystals. The photodiodes described overcome the area, sensitivity, and gain limitations of prior art photodiodes by employing a surface inversion layer to improve sensitivity, using improved growth and doping techniques to achieve a large uniform area, and using transmutated, doped silicon to reduce local voltage breakdown as described in those publications. The publications also describe bevelled edge surfaces of from five to fifteen degrees, to avoid premature breakdown at high voltage due to surface currents, as well as antireflection coatings for light detection enhancement.
The photodiodes described in those publications are formed by the diffusion of P-type material into a bulk N-type silicon wafer. The diffusion creates a graded concentration of dopants beginning at the wafer surface and extending inwardly. A graded dopant concentration produces a net drift of carriers towards a broad area PN junction formed in a plane parallel to the wafer surface. The drift region extends to the edge of the space charge region associated with the PN junction.
Under conditions of a reverse bias (1500 volts) across the PN junction, a very high electric field is produced creating an avalanche region extending from the drift region through the avalanche (space charge) region of the PN junction. Minority carriers (electrons) drift towards the avalanche region and enter the space charge region where they quickly attain velocities to cause collisions with bound electrons in the lattice of the wafer crystal. The collisions free new electrons which in turn undergo new collisions resulting in a net gain in the electrical signal.
The active thickness of the device is the drift region of about 200 micrometers with full gain being realized only by carriers generated in the first 10 to 60 micrometers. Because of the small active region and the known low stopping power of silicon, the photodiodes are largely insensitive to X-rays or Gamma rays of energies greater than 20 kev.